Linear actuators are used in many industrial products. In the aeronautical industry in particular are used as actuators of aircraft control surfaces and other aircraft components.
Generally, each aircraft control surface is actuated by multiple linear actuators in parallel, so that in case of power loss of one of them, the surface can be controlled with the remaining actuators. As this configuration has the disadvantage that the jamming of one of the actuators may produce a blocking of the surface, the aeronautic regulations require extremely low jamming probabilities (of the order of 10e-9) to said actuators. Hydraulic actuators are capable of meeting this requirement.
The trend toward greater electrification of aircraft (“More Electrical Aircraft”, MEA), oriented toward a reduction of weight and maintenance cost of systems, has led to the introduction of new technologies in flight command systems, including primary flight command systems.
Electro-hydrostatic actuators (EHAs) have been incorporated in new platforms (A380, A400, A350, F35, . . . ). This type of actuator has an integrated hydraulic system so that interconnection with the power system of the aircraft is purely electric, but its power transmission to the aircraft control surface is through an integrated hydraulic actuator. They meet the jamming probability target because the power transmitted to the surface is done by means of a hydraulic actuator and at the same time allows the elimination of the aircraft hydraulic system from the aircraft. This technology is considered an intermediate step in the progressive electrification of aircraft actuation systems.
Electro Mechanical Actuators (EMA) have not yet been implemented in primary flight command systems (except for experimental applications), despite their potential advantages with respect to complexity, efficiency, weight and maintainability. The main reasons why EMAs have not been introduced in primary flight command systems controls are:                The probability of jamming of current available actuators is not as low as required.        The reversibility of these actuators in case of loss of electrical power is not as good as in the case of actuators with a hydraulic output stage, especially if the mechanical advantage between the electric motor and the output is high.        
Commonly applied technologies in the output stage of EMA actuators (primarily ball screws and mechanical reduction gearbox) do not fully guarantee the above requirements because of:                The mechanical gearbox usually connected between the electric motor and the screw has a higher jamming probability than required for the application. The ball screw has the same problem by incorporating re-circulating mechanical elements, which, when blocked, impede or degrade the movement of the screw up to a non-functionality level.        The reversibility of the ball screw in case of jamming is low, because small pitches are commonly used to minimize the size of the electric motor.        
The planetary roller screws have advantages over ball screws with similar efficiency in terms of strength, life and load capacity among others. Their design is simpler and does not include circulation elements. However, they are not free of jamming in their moving parts (rollers, synchronism crown, gears, etc.) by the presence of external contamination, fractures, etc., thus jamming the output shaft of the actuator.
U.S. Pat. No. 7,410,132 and U.S. Pat. No. 7,610,828 disclose ball screw linear actuators incorporating means for releasing the output shaft in case of jamming.
One disadvantage of these proposals is that the ball screw has recirculating elements susceptible to jamming in the recirculation channel. They involve therefore a relatively high probability of jamming.
Another disadvantage is that in both proposals the unlocking of the output linear element is performed at the level of the nut of the screw. Then, after releasing the output linear element, a parallel actuator (in the above-mentioned case of a flight control surface actuated by a set of parallel actuators) should drag both the screw and the nut. This means that the actuator shall be designed leaving free the volume swept by the screw and the nut along the whole run of the parallel actuator. In addition, the inertia to be dragged by the parallel actuator is the inertia of the screw and the nut.
US Patent Application US2007/295125 discloses a linear actuator comprising:                a rotatory input shaft driven by an electric motor;        an output shaft having a helical threaded zone in its external surface at its inner end;        a first roller gear configured to rotate with respect to it axis when the input shaft rotates;        a plurality of second roller gears configured to engage with the first roller gear and with the output shaft in its helical threaded zone so that the rotation of the first roller gear is firstly transmitted to the second roller gears, which rotate with respect to their axis, and secondly converted in a linear movement of the output shaft.        